1. Field of the Invention
The present invention relates to a pump assembly for fuel, having a main feed pump and a preceding prefeed pump, which pumps a fuel flow via a fuel line out of a tank and the pumped total fuel flow through a drive/crank chamber of the main feed pump, wherein downstream of the drive/crank chamber in terms of the flow direction of the total fuel flow, a return line for the lubricant flow to the tank and a feed course for the pumping flow to a metering unit and on to the main feed pump are provided.
2. Description of the Prior Art
A pump assembly as described above is known for instance from German Patent Application 198 01 355. In it, a pump assembly for fuel is disclosed which has a main feed pump, embodied as a high-pressure pump, and a preceding prefeed pump. The prefeed pump is embodied as an electric fuel pump and pumps a fuel flow out of a tank via a fuel line. The pumped total fuel flow is carried through the drive/crank chamber of the main feed pump. Downstream of the drive/crank chamber, the total fuel flow is split into a lubricant flow and a pumping flow. The lubricant flow returns to the tank via a return line. The pumping flow, via a feed course, first reaches a metering unit and then goes on to reach the main feed pump.
The main feed pump has at least one pump cylinder, which defines a pump chamber in which a piston is guided so that it can reciprocate. By the reciprocation of the piston, in a so-called intake stroke, the pumping flow is aspirated into the pump chamber, and in an ensuing pumping stroke, the fuel located in the pump chamber is compressed and pumped for instance into a storage volume of a common rail injection system.
To effect the reciprocation of the pump in the pump chamber, the main feed pump of the known pump assembly also has drive means, embodied for instance as a drive shaft. The drive means rotate or move at high speeds within a so-called drive or crank chamber. Upon the rotation or motion of the drive means in the drive/crank chamber, a major thermal and/or mechanical load occurs in this region. The total fuel flow pumped by the prefeed pump is therefore carried entirely through the drive/crank chamber of the pump assembly. Because of the high fuel throughput in the region of the drive/crank chamber, especially good lubrication and major heat dissipation are made possible. In particular, forced lubrication of the drive/crank chamber is also possible, since the total fuel flow is present at the full pumping pressure of the prefeed pump in the drive/crank chamber.
The object of the present invention is to improve a pump assembly of the type recited at the outset, and in particular to create a pump assembly which is especially simple and sturdy in design and functions especially reliably.
To attain this object, the invention, based on the pump assembly of the type recited at the outset, proposes that the prefeed pump is mechanically driven, and that between the metering unit and the main feed pump, a zero-feed course branches off, in which a zero-feed throttle is disposed and which discharges into the fuel line on the intake side of the prefeed pump.
In the pump assembly of the invention, the prefeed pump is embodied as a mechanically driven pump of especially simple and sturdy design. However, first, the pump assembly has to be adapted to an insert of a mechanically driven prefeed pump. For this reason, the pump assembly of the invention has a zero-feed course, which branches off between the metering unit and the main feed pump, or in other words on the intake side of the main feed pump, and discharges into the fuel line upstream of the prefeed pump, that is, on the intake side of the prefeed pump. A zero-feed throttle is disposed in the zero-feed course, in order to limit a zero pumping flow to a certain flow rate. It is conceivable for the zero-feed course to discharge into the tank instead of into the fuel line. However, if a filter is disposed between the tank and the prefeed pump, then the zero-feed course should discharge into the fuel line between the filter and the prefeed pump.
In an advantageous refinement of the present invention, it is proposed that the prefeed pump is embodied as a gear pump. A gear pump is especially simple in design, functions reliably, and has especially high efficiency. For use in the hydraulics of motor vehicles, gear pumps are especially highly suitable. In the pump assembly of the invention, however, piston pumps, vane cell pumps, roller cell pumps, or other pumps can also be used as the prefeed pump.
In a preferred embodiment of the present invention, a check valve and a throttle are disposed one after the other in the return line.
Advantageously, parallel to the return line, a further return line, in which a venting throttle is disposed, is provided. The further return line is embodied for instance as a venting bore with a certain throttle cross section. If the pump assembly of the invention is used to pump fuel for an internal combustion engine of a motor vehicle, then because of the further return line, restarting the engine after the tank has been emptied is possible without any problems. The further return line also branches off downstream of the drive/crank chamber of the main feed pump, in the flow direction of the total flow and also discharges into the tank. The return line and the further return line can also be embodied as a common return line, in which the venting throttle on the one hand and the check valve and the throttle on the other are disposed parallel to one another.
In a further preferred embodiment of the invention, it is proposed that from the feed course, an overflow course branches off, in which an overflow valve is disposed and which discharges into the fuel line upstream, in the flow direction of the total fuel flow, of the prefeed pump. The overflow quantity of the overflow valve carries the remaining flow, that is, the quantity of fuel not carried on from the metering unit to the main feed pump, back to the intake side of the prefeed pump.
Advantageously, the metering unit is embodied as a proportional slide valve.
In another preferred embodiment of the invention, the main feed pump is embodied as a high-pressure pump, in particular as a multi-cylinder radial-piston eccentric pump. This kind of radial piston pump has a drive shaft supported rotatably about its longitudinal axis in a housing of the pump. The drive shaft is embodied eccentrically or is provided with camlike protrusions in the circumferential direction. The radial piston pump has a plurality of pump cylinders disposed radially to the drive shaft. Each of the pump cylinders defines one pump chamber, in which a piston is guided to reciprocate. By rotation of the drive shaft, the pistons are moved back and forth into the pump chambers.
The pump assembly of the invention preferably pumps the fuel into a storage volume of a common rail injection system. A common rail injection system is used to supply fuel to internal combustion engines. In contrast to conventional high-pressure injection systems, in which the fuel is pumped to the individual engine combustion chambers via separate lines, in common rail injection systems the injection nozzles are supplied from a common storage volume, which is also called a common rail. Especially in common rail injection systems, the advantages of the pump assembly of the invention are especially valuable, since the production costs for common rail injection systems can be reduced, and the reliability of these injection systems can improved decisively, by the use of the pump assembly of the invention.
In a preferred embodiment of the present invention, the main feed pump and the prefeed pump are combined into an integral pump unit. The pump unit is made to communicate toward the outside with the tank via the fuel line and a return line for the lubricant flow and for venting purposes, and via a high-pressure line, it is made to communicate with the storage volume of the common rail injection system. The requisite connections for the communications with the tank and the storage volume of the common rail injection system can all be made toward the outside on a common flange, where they can be joined especially simply to the corresponding connection lines. The integral pump unit can also be made with an especially small size.